1. Field of the Invention
This invention relates to protective coatings for stretched acrylic windows, and more particularly to protective coatings which maintain the optical quality and reduces crazing of the windows in use.
2. Background
Aircraft passenger windows are typically made of stretched acrylic (i.e. acrylic substrate) due to its light weight, flexibility and formability. However, acrylic is susceptible to particle (e.g. sand) and water induced erosion and crazing. Moreover, during flight, aircraft windows are subjected to differential pressures caused by the difference in pressure between the inside and the outside of the aircraft. The differential pressure causes the windows to flex, and therefore can cause most coatings to crack, allowing chemicals to attack the acrylic substrate and in some cases allowing the coating to delaminate from the acrylic substrate.
Windows can also be subjected to a variety of chemicals, such as detergents and other chemicals used in aircraft maintenance.
Presently, acrylic and other types of aircraft windows are protected by sol-gel based polysiloxane coatings. The sol-gel coatings are homogeneous mixtures of a solvent, an organosilane, alkoxide and a catalyst that are processed to form a suitable coating.
The sol-gel coatings provide high transmittance, but limited durability against wear and UV induced degradation. The sol-gel coatings exhibit only a moderate durability as judged from field results. This is evident in FIG. 1 where the hard coating is not present over a major portion of a window 100 taken off an airline that was in service for about four years. Extensive scratches and crazing 102 can also be seen.
Therefore, there is a need for a transparent, hard coating with improved durability that would extend window life time. The improved coating should also provide improved resilience against chemicals commonly encountered in aircraft maintenance and also improved weatherability characteristics.